Ceramic materials are of critical importance for a number of high temperature, high performance applications such as gas turbines. These applications require a unique combination of properties such as high specific strength, high temperature mechanical property retention, low thermal and electrical conductivity, hardness and wear resistance, and chemical inertness. Design reliability and the need for economical fabrication of complex shapes, however, have prevented ceramic materials from fulfilling their potential in these critical high temperature, high performance applications.
The design reliability problems with ceramics, and the resultant failure under stress, are due largely to the relativeIy brittle nature of ceramics. This, in combination with the high cost of fabricating complex shapes, has limited the usage of ceramics.
Ceramics made from organosilicon polymers have the potential to overcome these problems. To this end, polymers based on silicon, carbon and/or nitrogen and oxygen have been developed. See, e.g., "Siloxanes, Silanes and Silazanes in the Preparation of Ceramics and Glasses" by Wills, et al., and "Special Heat-Resisting Materials from Organometallic Polymers" by Yajima, in Ceramic Bulletin, Vol. 62, No. 8, pp. 893-915 (1983), and the references cited therein.
The major and most critical application for ceramics based on polymer processing is high strength, high modulus, shaped articles such as fibers. Such fibers are spun from organosilicon preceramic polymers, and then cured and pyrolyzed to their ceramic form. The low molecular weight and highly branched structure of typical preceramic polymers, however, alters the spinning and subsequent fiber handling behavior of these polymers from that of conventional polymers. In particular, gelation and foaming tendencies in the melted polymers used for melt spinning may lead to the presence of undesirable flaws in the resulting fiber. Such flaws are undesirable in fine diameter fibers since they are believed to be the source of cracking and lowered tensile strength. Furthermore, because of the low molecular weight of the preceramic polymers used, the fibers spun therefrom have relatively low tensile strength and are difficult to handle in spinning, curing, and subsequent pyrolysis operations.
One important step in the formation of shaped articles such as fibers involves curing the preceramic polymer prior to pyrolyzing the same. Although various curing techniques, such as oxidative/hydrolytic cures, are known in the art, nevertheless, a need exists for developing improved curing procedures which will render the fibers inert to morphological changes other than the desired densification as the filaments are pyrolyzed to ceramics.
Typical curing procedures of the prior art are described in U.S. Pat. Nos. 3,853,567, 4,535,007, and 4,399,232, wherein various curing agents are described.
The instant invention involves the use of gaseous thionyl chloride as a curing agent in order to obtain improved results including rapid curability, increased green strength (strength before pyrolysis) as well as increased strength of the shaped ceramic articles.
The expression "gaseous thionyl chloride" is intended to include thionyl chloride at temperatures sufficiently elevated to be in the gaseous state as well as saturated mixtures of thionyl chloride with an inert gas such as nitrogen or argon. In the latter case, the thionyl chloride can be below its boiling point.